libgomp: Handle OpenMP's reverse offloads

libgomp: Handle OpenMP's reverse offloads

This commit enabled reverse offload for nvptx such that gomp_target_rev
actually gets called.  And it fills the latter function to do all of
the following: finding the host function to the device func ptr and
copying the arguments to the host, processing the mapping/firstprivate,
calling the host function, copying back the data and freeing as needed.

The data handling is made easier by assuming that all host variables
either existed before (and are in the mapping) or that those are
devices variables not yet available on the host. Thus, the reverse
mapping can do without refcounts etc. Note that the spec disallows
inside a target region device-affecting constructs other than target
plus ancestor device-modifier and it also limits the clauses permitted
on this construct.

For the function addresses, an additional splay tree is used; for
the lookup of mapped variables, the existing splay-tree is used.
Unfortunately, its data structure requires a full walk of the tree;
Additionally, the just mapped variables are recorded in a separate
data structure an extra lookup. While the lookup is slow, assuming
that only few variables get mapped in each reverse offload construct
and that reverse offload is the exception and not performance critical,
this seems to be acceptable.

libgomp/ChangeLog:

	* libgomp.h (struct target_mem_desc): Predeclare; move
	below after 'reverse_splay_tree_node' and add rev_array
	member.
	(struct reverse_splay_tree_key_s, reverse_splay_compare): New.
	(reverse_splay_tree_node, reverse_splay_tree,
	reverse_splay_tree_key): New typedef.
	(struct gomp_device_descr): Add mem_map_rev member.
	* oacc-host.c (host_dispatch): NULL init .mem_map_rev.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_get_num_devices): Claim
	support for GOMP_REQUIRES_REVERSE_OFFLOAD.
	* splay-tree.h (splay_tree_callback_stop): New typedef; like
	splay_tree_callback but returning int not void.
	(splay_tree_foreach_lazy): Define; like splay_tree_foreach but
	taking splay_tree_callback_stop as argument.
	* splay-tree.c (splay_tree_foreach_internal_lazy,
	splay_tree_foreach_lazy): New; but early exit if callback returns
	nonzero.
	* target.c: Instatiate splay_tree_c with splay_tree_prefix 'reverse'.
	(gomp_map_lookup_rev): New.
	(gomp_load_image_to_device): Handle reverse-offload function
	lookup table.
	(gomp_unload_image_from_device): Free devicep->mem_map_rev.
	(struct gomp_splay_tree_rev_lookup_data, gomp_splay_tree_rev_lookup,
	gomp_map_rev_lookup, struct cpy_data, gomp_map_cdata_lookup_int,
	gomp_map_cdata_lookup): New auxiliary structs and functions for
	gomp_target_rev.
	(gomp_target_rev): Implement reverse offloading and its mapping.
	(gomp_target_init): Init current_device.mem_map_rev.root.
	* testsuite/libgomp.fortran/reverse-offload-2.f90: New test.
	* testsuite/libgomp.fortran/reverse-offload-3.f90: New test.
	* testsuite/libgomp.fortran/reverse-offload-4.f90: New test.
	* testsuite/libgomp.fortran/reverse-offload-5.f90: New test.
	* testsuite/libgomp.fortran/reverse-offload-5a.f90: New test without
	mapping of on-device allocated variables.